Reality is that the clothing you wear would make more of a difference on extending your season than the kayak. Whether you are in a sit on or sit in, you need to be wearing clothing that will keep you comfortable when you are paddling, and at the same time able to keep you alive if you swim.
Here are coastal water temperatures (reported and averages) for mid-Atlantic area: CWTG - NCEI Coastal Water Temperature Guide - Atlantic Coast: Central table The temps drop off pretty quickly as you get into fall.
The guidelines I follow is when the water temp is 60 or below, I am wearing some sort of thermal protection for a swim. That would mean a wet suit (usually farmer john, 3 mm, often with a paddle jacket added) or dry suit. Below 50 and I only would be in a dry suit. That said, the dry suit costs basically what you are looking to spend for a kayak…
Back to kayaks - here is a breakdown of the American Canoe Association’s ratings of categories based on conditions, and what boat would be appropriate:
Note that paddler skill isn’t talked about - just what level the boat can do. Of course, paddler skills must match the conditions in order for the boat to be able to handle a certain level.
In regards to sit in kayaks, that skill comes to play. First, the sit in kayak you need is a sea/touring kayak. The short and wide recreational class kayaks are only appropriate for level 1 conditions (definitely not coastal. as most don’t have flotation for deep water rescues). There is an article available online at California Kayaker Magazine - South West's source for paddlesports information (issue #10) that talks about different types of kayaks that might be worth a read to understand about types of kayaks.
With a touring kayak, getting back in after a flip takes a bit more time and skill/knowledge than a sit on top would, so a strong recommendation is to take at least an introduction to sea kayak class, that would include you doing a paddlefloat and T-rescue. That same magazine issue listed above has an article on T-rescues, if you want to see what is involved.
Marshall is ACA certified and knows all of this (and would be a good source for a class also). He will set you up well, if you go that way.
If you watch, you may see appropriate sit insides used (that said, with the pandemic things are all crazy in availability). Some boats that should fit you are Dagger Alchemy 14.0L, Dagger Stratos 14.5L, Jackson Journey 14, or P&H Delphin 155. Definitely get instruction or otherwise make sure you now how to wet exit and reenter the boat in deep water.
If you wanted to stick to sit on top but upgrade from what you have, I would look for one that is a bit longer and a little narrower than the Frenzy. Something more like 12’ long and with a capacity well in excess of 300lbs would be better.
